1. Technical Field
The present invention relates to battery chargers, and more particularly, to a charging-current controller for efficiently charging a rechargeable battery of a portable device where multiple external power sources are alternatively available.
2. Description of the Related Art
With the spread of mobile computing, portable personal computers (hereinafter referred to as portable PCs) of various sizes and functions have been developed. For example, there are notebook personal computers (PCs), sub-notebook PCs, palmtop PCs, PDAs (personal data assistants) or the like.
Portable PC's typically incorporate a battery within their main housing. This battery makes it possible for a user to use a portable PC in an environment where a line power source is not readily available. Typically, a rechargeable battery is used so that it may be charged and used repeatedly.
In an environment where a line power source can be utilized, the user connects an AC adapter (to which the line power alternating current is input and from which direct current is output) to the portable PC. With this power connection, it becomes possible for the portable PC to charge its incorporated secondary battery while the portable PC is operating.
Since the portable PC has limited space in its main housing, an expansion unit may be connected to the main housing to extend the functionality of the portable PC. This expansion unit is typically referred to as a docking station, a port replicator or the like.
Some expansion units incorporate a power source that may be driven by a line power source. If a portable PC is docked to an expansion unit incorporating a power source, the portable PC can take advantage of the power source incorporated in the expansion unit.
When an AC adapter is being connected to a portable PC, the rechargeable battery is charged by utilizing the AC adapter. When a portable PC is being docked to an expansion unit, the rechargeable battery is charged by utilizing the power source incorporated in the expansion unit.
An AC adapter is made small in size and light in weight because it is usually carried with a portable PC. The DC power that is output by the AC adapter, therefore, is restricted. On the other hand, an expansion unit can be made larger because it is usually placed on a desk to be used. Therefore, because the power source incorporated in the expansion unit can be made larger than the AC adapter, the expansion unit can output a higher DC current than that of the AC adapter.
Typically, a charger for charging the rechargeable battery incorporated in the portable PC is formed to conform to the AC adapter. For this reason, even when a portable PC is docked to the expansion unit so that it can receive a higher charging current than available from an AC adapter, a current for charging the rechargeable battery will be limited to the same value as provided by the AC adapter. Thus, even if a power source capable of supplying a larger power were connected to the portable PC, the larger power cannot be utilized to charge the rechargeable battery.
The present invention has been accomplished in order to solve the above-mentioned problem. Accordingly, it is an object of the present invention to provide a charging-current controller that is capable of efficiently charging a rechargeable battery in conformance with the capacity of a power source to be connected to a portable PC.